1. Field of the Invention
The present invention relates to an ion-mobility spectrometer and an ion-mobility analysis method for analyzing ions ionized at an ion source.
2. Description of the Related Art
Ion mobility spectrometry has been widely used for gas detection such as explosive detection. In addition to ion detection method based on the ion mobility, a mass spectrometry method exists as ion detection method. In the ion mobility spectrometry, ion separation is performed by the ion mobility differences. Meanwhile, in the mass analysis method, the ion separation is performed by mass-to-charge ratios. Namely, these two methods differ fundamentally. In the ion mobility method, the separation is performed under a pressure of 10 mTorr or higher, where ions collide with gas molecule many times during the ion-separation time. This means that this method positively utilizes collision effects between the ions and the gas molecule. Meanwhile, in the mass analysis method, the separation is performed under a pressure of 1 mTorr or lower, where the collisions between the ions and the gas molecule are small in number during the ion-separation time.
In JP-A-2004-504696, the ion mobility method has been described in detail. According to the description in JP-A-2004-504696, assuming that electric field is constant, reach time T needed to go through the flight distance, L, is represented by (Expression 1). Here, let the ion mobility be K, voltage be V, and displacement distance be L, respectively.T=L2/(KV)  (Expression 1)
Depending on ion species, values of the ion mobility K differ from each other. This makes it possible to separate the ion species by using reach times of the flight to the detector. The ion mobility has been widely utilized for such apparatuses as an explosives detection apparatus at an airport or the like.
In JP-A-2004-504696, the following method has been described. Namely, after separating the ions by using the ion mobility as described, ion dissociation is performed in a reaction chamber, and then the fragment ions after being dissociated are detected at a mass spectrometer such as time-of-flight mass spectrometer that operate in high-vacuum pressure. According to the description in JP-A-2004-504696, after the ions have been once separated by using the ion mobility, the ions separated are sequentially introduced into a reaction chamber such as a collision cell. Then, the ions introduced into the reaction chamber are sequentially introduced via collision dissociation or the like into the mass analysis unit such as a time-of-flight mass spectrometer. Here, it is possible to acquire two-dimension-mannered data (i.e., the first dimension: mass mobility by the ion mobility spectrometry of the ions before being dissociated, the second dimension: mass-to-charge ratio on the ions after being dissociated), which enhances the resolving power tremendously. An ion separation time by the ion mobility spectrometry is equal to about tens of ms (peak width: from a few hundreds us to a few ms). In contrast thereto, a time needed for acquiring mass spectrum by TOF mass spectrometer in high vacuum is equal to 100 μs or less. This separation time difference allows each mass spectrum to correspond to some species separated by the ion mobility.
In U.S. Pat. No. 6,348,688, the following method has been described. Namely, after separating the ions by using the time-of-flight mass spectrometer, only specific ions are separated by voltage switching. After that, the specific ions separated are introduced into a collision-dissociation chamber, where ions are dissociated and convert fragment ions. Then, the fragment ions are subjected to the time-of-flight mass spectrometer once again. According to the description in U.S. Pat. No. 6,348,688, it is possible to acquire exceedingly high selectivity and abundant data (first-stage: mass-to-charge ratio before the dissociation by mass spectrometry, second-stage: mass-to-charge ratio after the dissociation by mass spectrometry).